It takes Mary 30 minutes to walk uphill 1 \mathrm{~km} from her home to school, but it takes her only 10 minutes to walk from school to home along the same route. What is her average speed, in \mathrm{km} / \mathrm{hr}, for the round trip?
Answer Choices
A. 3
B. 3.125
C. 3.5
D. 4
E. 4.5